Direct heating cathode structure for electron tubes

ABSTRACT

A direct heating cathode structure for electron tubes, which has a low electrical conductivity cermet layer, an intermediate metal layer of a material capable of promoting the electron emission of electron emissive materials and an electron emissive material layer successively laminated on a cathode body of a high resistivity alloy. The cermet layer in this cathode structure provides improved heat generating effect of heater current on the cathode body, permits passage of adequate current into the intermediate metal layer and electron emissive material layer, and prolongs the service life of the cathode.

United States Patent 1 Misumi 11 3,745,403 [4 1 July 10,1973

[ DIRECT HEATING CATHODE STRUCTURE FOR ELECTRON TUBES [75] Inventor: Akira Misumi, Mobara-shi, Japan [73] Assignee: Hitachi, Ltd., Tokyo, Japan [22] Filed: Nov. 30, 1971 21] Appl. No.: 203,386

[52] US. Cl 313/341, 313/217, 313/218, 313/310, 313/345, 313/355 [51] Int. Cl H01j 1/15, HOlj 19/06 [58] Field of Search 313/345, 346, 355, 313/310, 217, 218, 341

[56] 1 References Cited UNITED STATES PATENTS 2,172,207 9/1939 Kolligs et al. 313/341 Primary Examiner-David Schonberg Assistant ExaminerPaul A. Sacher Attorney-Paul M. Craig, Jr. et a1.

[57] ABSTRACT 10 Claims, 5 Drawing Figures 1 DIRECT HEATING CATHQDE STRUCTURE FOR ELECTRON TUBES This invention relates to direct heating cathode structures for electron tubes and, more particularly, to improvements in the direct heating cathode structure having an intermediate metal layer and an electron emissive layer successively laminated on a cathode body.

In the usual direct heating cathode, the electron emissive material is directly deposited on the heating element. Because electron emission is immediately obtained upon energization of the direct heating cathode, it is extensively used in such electron tubes as cathoderay tubes and reception tubes.

An object of the invention is to provide a direct heating cathode structure for electron tubes, which can overcome the drawbacks in the prior-art direct heating cathode structure to be described hereinafter, and which has a cermet layer, an intermediate metal layer and an electron emissive layer successively laminated on a cathode body.

More specifically, there is featured by the invention a direct heating cathode structure for electron tubes, which comprises a cathode body of a high resistivity alloy, a high resistivity cermet layer provided on said cathode body, an intermediate metal layer provided on said cermet layer and capable of promoting the electron emission of electron emissive materials, and an electron emissive layer provided on said intermediate metal layer.

The above and other objects, features and advantages of the invention will become more apparent from the following description when read with reference to the accompanying drawing, in which;

FIGS. 1 and 2 are sectional views showing examples of the conventional direct heating cathode structure for electron tubes;

FIG. 3 is a sectional view for electron tubes embodying the invention; and

FIG. 4a is a perspective plan view of a coil-like cathode body and FIG. 4b is a cross-section of FIG. 4a taken along line IVB--IVB where reference numerals correspond to those of similar parts shown in FIG. 3.

Before describing a preferred embodiment of the invention, prior-art direct heating cathode structures that constitute the background of the invention will first be discussed with reference to FIGS. 1 and 2.

FIG. 1 shows an example of the prior-art direct heating cathode. The illustrated cathode comprises asubstantially U-shaped cathode body 1, which is made of pure metal such as nickel and tungsten or a high resistivity alloy such as nicrome and hastelloy (a trade name). The cathode body 1 has its legs la and lb connected to respective cathode terminal stems 2a and 2b. The upper face of the central top of the cathode body I is covered with an electron emissive material 3 for improving the electron emissive characteristics of the cathode.

With this direct heating cathode structure where the electron emissive material 3 is'directly deposited on the cathode body 1 which serves as heating element, emission of electrons takes place immediately when the cathode body 1 is energized. This isan advantage in that prompt picture display may be obtained when the cathode is used for cathode-ray tubes, for example. However, if the afore-mentioned pure metal such as nickel and tungsten having high electrical and thermal conductivities is used. for the cathode body 1, low volt age and large current would be required to provide heating power, so that the heater current varies greatly with slight fluctuations of the contact resistance such as plug resistance, resulting in unstable temperature of the cathode body 1. Also, the temperature distribution in the electron emissive material 3 tends to be nonuniform due to cooling effect on the cathode terminal side. Therefore, density and initial speed of the emitted electrons vary greatly, so that uniform control of emitted electrons by grid electrodes cannot be obtained. This is disadvantageous, for instance, in that clear picture image cannot be obtained where the cathode is used for cathode-ray tubes. On the other hand, using a high resistivity alloy such as nicrome and hastelloy for the cathode body 1 would result in inferior electron emission characteristics since such alloy is incapable of causing the electron emissive material 3 to emit enough electrons.

FIG. 2 shows another cathode structure for electron tubes which has been proposed to solve the above problems. In this direct heating cathode structure, an intermediate metal layer 4 composed of such material as nickel and containing an element capable of promoting the electron emission of the electron emissive material 3, for instance magnesium, is provided between high resistivity alloy cathode body 1 and electron emissive material 3. This structure, however, has a drawback in that in the course of use of the cathode the intermediate metal layer 4 is deteriorated due to mutual diffusion that takes place between the cathode body 1 and intermediate metal layer 4, so that the electron emission is gradually deteriorated. Also, since the electrical conductivity of the intermediate metal layer 4 is higher than that of the cathode body 1, in that part of the cathode in which the intermediate metal layer 4 is present almost all the heating current flows in the intermediate metal layer 4. Therefore, no substantial heat is generated in part of the cathode body 1 contacting to the intermediate metal layer 4, and the electron emissive material 3 is heated through heat conduction from the other part of the cathode body 1, so that the prompt emission character which is the most important feature of the direct heating cathode is extremely deteriorated.

To overcome the above drawbacks, it may be considered to provide a thin insulating layer (for instance an alumina layer) between the cathode body 1 and intermediate metal layer 4, so that the insulating layer may serve to preventthe heating current from flowing into the intermediate metal layer 4. With such structure, however, separate electrodes would be required for supplying electrons to the electron emissive material 3. Also, thermal conductivity would be degradated by the insulating layer.

The present invention has been intended to cope with the above'shortcomings, and its preferred embodiment willnow be described with reference to FIG. 3, where the same parts as'those shown in FIGS. 1 and 2 are designated by identical reference numerals.

The embodiment of FIG. 3 is different from the structure of FIG. 2 in the provision of a cermet layer 5 composed of various heat-resisting, electrically insulating compounds such as oxides, carbides and nit-rides and various metals and having a comparatively low electrical conductivity. The cermet layer 5 intervenes between intermediate metal layer 4 and cathode body 1.

A desired electrical conductivity of the cermet layer 5 may be obtained by appropriately adjusting the composition thereof. The electrical conductivity of the cermet layer 5 is adjusted such that only an extremely small part (for instance 1 to 5 percent) of the heater current flows through the cermet layer 5 while the other part of the heater current flows through the cathode body 1 for heat generation. In other words, even with cath ode body 1 of a high resistivity alloy such as nicrome and hastelloy most part of the heater current will flow through the cathode body 1 to generate sufficient heat in the close proximity of the intermediate layers, since the electrical conductivity of the cermet layer is lower than that of the high resistivity alloy.

While in the preceding embodiment of the cathode structure the cermet layer, intermediate metal layer and electron emissive layer are successively laminated on only the upper face of the central top of the U- shaped cathode body, the invention is by no means limited to this structure, but the individual layers may of course be laminated over the entire surfaces of a coilshaped cathode body, for example as shown in FIGS. 4a and 4b.

As has been described in the foregoing, since the direct heating cathode structure for electron tubes according to the invention has a low electrical conductivity cermet layer, an intermediate metal layer to promote the electron emission of electron emissive material and an electron emissive material layer, said respective layers being successively laminated on a high resistivity cathode body, the heater current can he mostly confined within the cathode body to improve the heat generating effect. Also, by virtue of the low conductivity cermet layer intervening between the cathode body and intermediate metal layer, an adequate part of the heater current flows through the cermet layer and intermediate metal layer into the electron emissive layer, so that no separate electrodes for supplying electrons to the intermediate metal layer are required. Further, by appropriately selecting the composition of the cermet layer, which may be constituted by a combination of such compounds as oxides, carbides and nitrides and various metal elements, the reaction between the cathode body and intermediate metal layer materials may be prevented to prolong the service life of the cathode.

Whatis claimed is:

1. A direct heating cathode structure for electron tubes, which comprises a cathode body of a high resistivity alloy, a high resistivity cermet layer provided on said cathode body, an intermediate metal layer provided on said cermet layer and capable of promoting the electron emission of electron emissive materials, and an electron emissive layer provided on said intermediate metal layer.

2. The direct heating cathode structure according to claim 1, wherein said cathodes body has a U-shaped configuration, and wherein said cermet layer, said intermediate metal layer and said electron emissive layer are successively laminated atop a central portion of said U-shaped cathode body.

3. The direct heating cathode structure according to claim 1, wherein said cathode body has a coil-like configuration, and wherein said cermet layer, said intermediate metal layer and said electron emissive layer are successively laminated over the entire surface of said coil-like cathode body.

4. The direct heating cathode structure according to claim 1, wherein the electrical conductivity of said cermet layer is lower than the electrical conductivity of the high resistivity alloy of said cathode body.

5. The direct heating cathode structure according to claim 1, wherein said cathode body is made of nicrome.

6. The direct heating cathode structure according to claim 1, wherein said cathode body is made of hastelloy.

7. The direct heating cathode structure according to claim 1, wherein said cathode body is made of nickel.

8. The direct heating cathode structure according to claim 1, wherein said cathode body is made of tungsten.

9. The direct heating cathode structure according to claim 1, wherein said intermediate metal layer is composed of nickel and contains magnesium.

10. The direct heating cathode structure according to claim 1, wherein said cermet layer has electrical conductivity adjusted so that a heater current flows through the cermet layer in an amount of 1 to 5 percent thereof. 

2. The direct heating cathode structure according to claim 1, wherein said cathodes body has a U-shaped configuration, and wherein said cermet layer, said intermediate metal layer and said electron emissive layer are successively laminated atop a central portion of said U-shaped cathode body.
 3. The direct heating cathode structure according to claim 1, wherein said cathode body has a coil-like configuration, and wherein said cermet layer, said intermediate metal layer and said electron emissive layer are successively laminated over the entire surface of said coil-like cathode body.
 4. The direct heating cathode structure according to claim 1, wherein the electrical conductivity of said cermet layer is lower than the electrical conductivity of the high resistivity alloy of said cathode body.
 5. The direct heating cathode structure according to claim 1, wherein said cathode body is made of nicrome.
 6. The direct heating cathode structure according to claim 1, wherein said cathode body is made of ''''hastelloy''''.
 7. The direct heating cathode structure according to claim 1, wherein said cathode body is made of nickel.
 8. The direct heating cathode structure according to claim 1, wherein said cathode body is made of tungsten.
 9. The direct heating cathode structure according to claim 1, wherein said intermediate metal layer is composed of nickel and contains magnesium.
 10. The direct heating cathode structure according to claim 1, wherein said cermet layer has electrical conductivity adjusted so that a heater current flows through the cermet layer in an amount of 1 to 5 percent thereof. 